Nowadays, objects are irradiated in many different realms of technology. A wide array of irradiation methods as well as a wide array of types of radiation are employed, depending on the concrete requirements of the application.
Thus, for instance, in some areas of technology, it is desirable to irradiate objects over the entire surface area and as uniformly as possible.
In other areas of technology, in contrast, specific parts of the object to be irradiated have to be irradiated with a specific, as a rule particularly high, dose while the other parts of the object are not irradiated at all or else only to a slight extent. An example of this is the structuring of microprocessors using electromagnetic radiation (in some cases, all the way into the X-ray range) as well as the structuring of imaging masks.
In yet other areas of technology, the dose distribution of the irradiation has to be structured not only in a two-dimensional plane but rather, in all three spatial dimensions. If applicable, a three-dimensionally structured irradiation with time variation has to be carried out (so-called four-dimensional structured irradiation). Such irradiation methods make it possible to introduce a specific, relatively high dose into a specific volume area located within an object that is to be irradiated. The area that surrounds the target volume that is to be irradiated, in contrast, can be exposed to a comparatively low dose. Examples of such a treatment of objects are found in the material sciences, in the manufacture of highly integrated components (especially microprocessors and memory chips) as well as in the production of nanostructured mechanisms.
Another field of technology that makes use of such three-dimensional or four-dimensional irradiation methods is that of medical technology. Here, it is likewise desirable to expose specific volume areas of the body, for instance, tumors, to the highest possible dose, whereas the surrounding tissue should only be exposed to the radiation dose to the smallest extent possible, or preferably not at all. This is particularly the case when the surrounding tissue is tissue such as, for example, one or more critical organs (usually referred to in technical terminology as OAR, short for “organ at risk”). Such a critical tissue can be, for instance, the spinal cord, major blood vessels (e.g. the aorta) or nerve nodes.
Such a desired selective irradiation “into the depths” of the object to irradiated can be achieved, for example, in that the irradiation takes place from many different directions, whereby all of the beams coming from different directions intersect at a specific point or in a specific target zone of the object to be irradiated. This translates into a high total dose at the point of intersection of the differently aimed beams, while the dose outside of this point of intersection is relatively low.
Another approach for achieving such a selective irradiation “into the depths” of an object consists of selecting certain types of radiation that, while passing through matter, display an energy-loss characteristic that has a peak that is as pronounced as possible. Examples of this are especially protons, ions and heavy ions. As they pass through matter, these types of radiation initially exhibit a relatively low energy loss per unit of length, so that the radiation dose deposited there is relatively low. On the contrary, most of the radiation energy is deposited in the so-called Bragg peak, so that the dose introduced into the object that is to be irradiated is very high there. As a result, a relatively sharply delineated target point can be reached “in the center” of an object that is to be irradiated. The dimensions of such a target point (or of a certain volume element, referred to in technical terminology as a “voxel”) can be, for example, within the range of a mere 1 mm3. A target volume to be irradiated having a specific contour can be selectively irradiated, for instance, by scanning methods, preferably by means of so-called raster scanning methods. Here, the target volume can be divided into so-called raster points. In this process, the particle beam (taking into consideration the Bragg peak) is passed successively over the target volume that is to be irradiated. A deflection in the X-Y plane (a so-called isoenergy plane) can be traversed by scanner magnets that can laterally deflect the particle beam. The depth can be varied by changing the energy of the particle beam, thus repositioning the Bragg peak. Whereas in the case of “classic” scanning methods, the particle beam is moved essentially continuously over the target volume, with the raster scanning method, the particle beam is always aimed at a raster point or voxel, where it remains for a certain period of time. As soon as a specific dose has been introduced into the voxel in question, the particle beam moves on to the next voxel.
Although the dose introduced into the object can be restricted relatively well to a certain volume area when heavy ions are employed, here as well, it is unavoidable that matter which is located in front of or behind the target point, especially along the particle beam, and which should actually not be irradiated, is exposed to a certain dose. This is particularly the case for regions that are in the front of the target area that is to be irradiated.
As a rule, the planning of the irradiation using currently available methods calls for the protection of organs at risk. The influence of movement that occurs during the irradiation, however, often cannot be adequately predicted. Consequently, when movement exerts an influence, the actual dose deposition in the target volume and/or especially in organs at risk can only be evaluated after the fact, if at all. Intervention is only possible with respect to fractions that might still follow. This, however, is problematic, particularly if the tumor is located in a critical tissue area (for instance, in an organ such as the lung or the heart), or if there is critical tissue in the immediate vicinity of the tumor. After all, with “normal” tissue, it is, in fact, possible to accept a certain, actually unnecessary, destruction of tissue, whereas in the case of such critical tissue (OAR), any damage should absolutely be avoided. In the past, this has often led to situations in which such tumors located near critical tissue could not be treated at all, or else at best only with severe side effects.
Especially problematic was the irradiation of target volumes in or near sensitive matter areas such as, for example, tumors in or near critical tissue, especially when the object in question moves, particularly when it moves of its own accord. Here, the matter surrounding the target volume to be irradiated can be moved not only translatorially, but also, and especially, rotatorially and/or deformatorially. As a consequence, for instance, if the beam position relative to the target volume or to the volume to be protected changes (due to the scanning method or also due to a change in the direction of the incident beam, for example, when a gantry is employed), the matter surrounding the target volume can be irradiated, for example, due to the movement “of its own accord” in the area of the incident particle beam; in particular, it might be irradiated differently than was intended in the preceding irradiation planning. This can cause a certain amount of damage to the affected tissue (the affected matter), which is particularly problematic if it is, for instance, a tissue that is to be specially protected (e.g. an OAR).